cp: show error if source & dest are same file

src/uu/cp/src/cp.rs

@@ -1713,6 +1713,7 @@ fn copy_file(
         if are_hardlinks_to_same_file(source, dest)
             && !options.force()
             && options.backup == BackupMode::NoBackup
+            && source != dest
         {
             return Ok(());
         }

tests/by-util/test_cp.rs

@@ -117,6 +117,20 @@ fn test_cp_duplicate_files() {
     assert_eq!(at.read(TEST_COPY_TO_FOLDER_FILE), "Hello, World!\n");
 }
 
+#[test]
+fn test_cp_same_file() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    let file = "a";
+
+    at.touch(file);
+
+    ucmd.arg(file)
+        .arg(file)
+        .fails()
+        .code_is(1)
+        .stderr_contains(format!("'{file}' and '{file}' are the same file"));
+}
+
 #[test]
 fn test_cp_multiple_files_target_is_file() {
     new_ucmd!()